The present invention relates to a shaped article of vinylidene fluoride resin having Young's modulus of not less than 450 kg/mm.sup.2 and a birefringence of not less than 30.times.10.sup.-3, and relates to a process for preparing a filament of vinylidene fluoride resin, comprising the steps of melt-extruding a vinylidene fluoride resin having an inherent viscosity of 0.85 dl/g to 1.4 dl/g and of Huggins' constant of not more than 0.45 at a shear rate of not more than 200 sec.sup.-1 and at a temperature of higher than the melting point of the vinylidene fluoride resin, subjecting the thus melt-extruded resin to drafting at a draft ratio (R) represented by the following formula: ##EQU3## wherein .eta..sub.app is an apparent melt-viscosity of the resin melt-extruded through a nozzle of 1 mm in diameter and 3 mm in length of a Melt-indexer at the same temperature as in the melt-extruding step, in a molten state thereby orienting the molecular chains of the resin to the extent that the birefringence of the thus treated resin becomes to not less than 30.times.10.sup.-3, and cooling the thus drafted resin and to a process for preparing a film of vinylidene fluoride resin, comprising the steps of melt-extruding a vinylidene fluoride resin having a melt-viscosity of 0.85 dl/g to 1.40 dl/g and Huggins' constant of not more than 0.45 from a T-die at a shear rate of not more than 200 sec.sup.-1 and at a temperature of higher than the melting point of the vinylidene fluoride resin, subjecting the thus melt-extruded vinylidene fluoride resin to drafting at a draft ratio of higher than 50, while retaining the distance between the outlet of said T-die and the center line of a roll to which the melt-extruded vinylidene fluoride resin contacts at the first time less than 10 cm, and cooling said vinylidene fluoride resin thus subjected to drafting to room temperature.
Namely, the object of the present invention is to provide a shaped article of a vinylidene fluoride resin, which is excellent in transparency and surface texture and has Young's modulus of not less than 450 kg/mm.sup.2 and a process for preparing the shaped article.
The hitherto-known value of Young's modulus of the filament of vinylidene fluoride resin has been at most 250 to 300 kg/mm.sup.2, and that of the film of vinylidene fluoride resin has been at most 100 to 140 kg/mm in a non-stretched state and at most 180 to 260 kg/mm.sup.2 in a stretched state thereof.
Young's modulus mentioned in the present invention is the initial modulus shown by the specimen of the shaped article of 100 mm in length and in the case where the specimen is a film, 20 mm in width, when the specimen is subjected to tensile stress into the drafting direction at a tensile velocity of 10 mm/min at 23.degree. C. on a tensile tester.
The conventional filament or the film of Young's modulus of such an extent as shown above is insufficient for various uses. For instance, when such a filament is used in angling, a good catch is not available because of poor communication of a bite via the filament, and as the ordinary fibers, the conventional filament of vinylidene fluoride resin is poor in stiffness. Further, as the strings of electronic musical instruments and the guts of tennis rackets, such a filament of vinylidene fluoride resin is poor in resilience and accordingly, it cannot make them exhibit their own performances.
In addition, in the case where the vinylidene fluoride resin is applied to window panes of a sun-bathing room while utilizing its property of transmitting ultraviolet rays of wave length in a range of 2800 to 3300 .ANG. (so-called Dorno rays) which are not transmitted by the ordinary glass pane, the healthy ultraviolet rays can be available, however, since the conventional film of vinylidene fluoride resin is poor in Young's modulus, it is apt to be deformed or slackened even by a slight external stress and accordingly, a very careful handling has been necessary to apply the film of vinylidene fluoride resin to such an use.
Further, since the vinylidene fluoride resin is excellent in weatherproofness, a large demand will be expected broadly in various outdoor uses, for instance, as building material, industrial material, transporting material, etc. in the event of improving its Young's modulus. In addition, since the strength of vinylidene fluoride resin is scarcely reduced in water, the use thereof in the marine development is greatly expected in the even of improving its Young's modulus.
Besides, the elongation at break of the conventional shaped articles of vinylidene fluoride resin is a relatively large, the smallest one having been 20% and accordingly, it is another object of the present invention to provide a shaped article of vinylidene fluoride resin, which has a smaller elongation even under a large stress and further, a sufficiently large tensile strength.
The present inventor, in consideration that the polyester fiber or the polypropylene fiber of high Young's modulus has hitherto been obtained by high-speed melt-spinning, that is, rapidly cooling the molten polyester or polypropylene while giving thereto a large shear strain, thereby orienting the polymer molecular chains and crystallizing thereof had an idea that a shaped article of vinylidene fluoride resin of a high Young's modulus may be obtained by the same process as above. However, it has been actually found by the present inventor that different from the case of polyester fiber or polypropylene fiber, in the case of vinylidene fluoride resin, a shaped article of vinylidene fluoride resin of such a high Young's modulus can be obtained only under the severely restricted conditions of melt-viscosity of vinylidene fluoride resin and the extent of branching of the polymer molecule thereof.
In addition, different from the filament of polyester or polypropylene, the film thereof of a high Young's modulus has not been obtained by the above-mentioned process as well as the film of vinylidene fluoride resin while retaining the shear rate in the melt-extruding smaller than in the case of polyester fiber or polypropylene fiber.
It has been found by the present inventor's studies that different from the case of filament, the film of vinylidene fluoride resin of the demanded high Young's modulus is obtainable only in the case when the melt-extruded resin is subjected to drafting at a closely restricted position very near to the outlet of the extruding die.
The present invention has been attained based on the above-mentioned findings of the present inventor.